Multi-bit cryptographically secure encryptor for m-ary spectral phase encoder  optical code division multiple access

ABSTRACT

A system and methods are provided for transmitting an encrypted data word of two or more bits. This involves identifying a random key word comprising two or more bits for encrypting the data word and identifying a set of unique orthogonal codes. This also involves selecting a code from the set of unique orthogonal codes that corresponds to a result of an exclusive-or (XOR) operation between the two or more bits of the data word and the random key word. This also involves encoding a signal with the code and transmitting the encoded signal as encrypted data.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Funding for research was partially provided by the Defense AdvancedResearch Projects Agency under federal contract MDA972-03-C-0078. Thefederal government has certain rights in this invention.

BACKGROUND

1. Technical Field

The present invention relates to signal processing and data encryption.In particular, the disclosed embodiments relate to communicating anencrypted data word via an optical code division multiplexed (OCDM)signal.

2. Description of the Related Art

Spectral phase encoding (SPE) involves selectively changing the phasesof the frequency lines in an optical pulse according to a particularcode. For example, if an optical pulse has frequency lines {f₁, . . .f_(n)}, then the frequency lines may be coded by a code {c₁ . . . .c_(m)}. An element in the code corresponds to a frequency line, and mayshift the phase of the frequency line according to a value of theelement. Different codes cause different shifts in the frequency lines.

Different codes may also map to different transmission data. Forexample, a code A (representing a series of frequency line phase shifts)may represent a data bit of 0 for transmission and a second code B(representing a different series of frequency line phase shifts) mayrepresent a data bit of 1 for transmission. Thus, an optical pulseencoded with code A may carry a value of 0 and an optical pulse encodedwith code B may carry a value of 1. It is assumed that a receiver knowsthe mapping between codes and transmission data, and therefore canextract the transmission data for each received coded optical pulse.This use of different codes to transmit data, rather than modulating theamplitude or phase of the optical pulses, is generally known ascode-shift keying (CSK) modulation.

The encoded signal, however, may be intercepted by a third party, whomay read the encoded optical pulse and determine whether code A or codeB was used for encoding. After intercepting enough signals, it would betrivial for the third party to determine that code A corresponds to 0and code B corresponds to 1.

Researchers at Princeton University devised a system for encoding anoptical pulse with one of a pair of codes that would not reveal thecorresponding data bit to an intercepting third party. FIG. 1illustrates this system.

FIG. 1 illustrates a system 100 for encoding an optical pulse with oneor the other of a pair of codes to carry a single bit of data. System100 includes transmitter 102 for encoding and transmitting an opticalpulse over link 104 and receiver 106 for receiving and decoding theoptical pulse. System 100 also includes interfering user i 108 andinterfering user j 110 that have access to link 104.

Transmitter 102 includes Optical Code Division Multiple Access (OCDMA)encoder A 112 and OCDMA encoder B 114. The OCDMA encoders operate bygiving physical delays to different wavelength components in an opticalsignals, and are not spectral phase encoders. OCDMA encoder A 112 andOCDMA encoder B 114 both receive optical pulses from a laser (notshown). OCDMA encoder A 112 encodes the optical pulses with a code A,while OCDMA encoder B 114 encodes the optical pulses with a code B.Instead of transmitting code A to represent a 1 and code B to representa 0, transmitter 102 encrypts the codes according to a random key bit.The key bit may be randomly generated or pseudo-randomly generated withequal probability of being 1 or 0. Thus, the code used to encode anoptical pulse is determined both according to the data bit and the keybit, as shown below.

Transmitter 102 also includes switches 116 and 118, both 2×2 switcheswith two inputs and two outputs. Switch 116 is controlled by a data bitfrom a plurality of data bits for transmission, while switch 118 iscontrolled by a random key bit from a plurality of random key bits forencrypting. In general, a value of 0 controls a switch to enter into a“bar” state, in which the upper input is switched to the upper outputand the lower input is switched to the lower output. Moreover, a valueof 1 controls a switch to enter into a “cross” state, in which the upperinput is switched to the lower output and the lower input is switched tothe upper output.

Switches 116 and 118 collectively perform an exclusive-or (XOR)operation between a single data bit and a single key bit to select acode. For example, if the data bit is 1 and the key bit is 1, then (1XOR 1)=0, and code A corresponding to 0 is selected for transmission.This is implemented by switches 116 and 118 as follows. Data bit 1causes switch 116 to enter into a cross state, so that a code A signal(outputted from OCDMA encoder A 112) is switched to a lower output ofswitch 116, and that a code B signal (outputting from OCDMA encoder B114) is switched to a upper output of switch 116. Code A and B signalsreach switch 118, which is controlled to enter into a cross state by keybit 1. Thus, the code A signal A is switched from the lower input ofswitch 118 to the upper output, and code B signal B is switched from theupper input of switch 118 to the lower output. Switch 118 drops itslower output and sends its upper output (i.e., the code A signal) onwardvia link 104. Thus, switches 116 and 118 perform an XOR operation ofdata bit 1 and key bit 1 by selecting code A.

The use of data bit 1 and key bit 1 is exemplary only. Indeed, table 120illustrates the resulting code transmissions arising from the fourdifferent combinations of data bits (d) and random key bits (r).

In this example, an optical pulse encoded with code A travels over link104 where it may be combined with signals from other users sharing link104, such as interfering user i 108 and/or interfering user j 110. Theinterfering users use different codes from A and B in their signals.More importantly, interfering user i 108 and interfering user j 110 (andany eavesdroppers on the system) will also receive a copy of thecombined signals on link 104 from all other users. This makeseavesdropping an issue. However, even if interfering user i 108 andinterfering user j 110 determine that the optical pulse is encoded withcode A, it is impossible to associate a data bit with code A withoutknowledge of the corresponding random key bit. Indeed, because code Awas selected by XORing the data word with a random key word, there is anequal likelihood that code A corresponds to either a 1 or a 0.

The optical pulse encoded with code A (combined with other signals fromother users) travels to receiver 106, which includes another 2×2 switch,switch 122. Switch 122 is controlled by the same key bit used to encodethe optical signal at transmitter 102. That is to say, the receiver hasto have knowledge of the random key bit string generated at thetransmitter. In this sense, this system corresponds to a one-time pad inwhich both the transmitter and receiver share a random key string thatis unknown to potential eavesdroppers. The key bit at receiver 106undoes the XOR at transmitter 102, so that receiver 106 can correctlyrecover the data bit. In particular, switch 122 is controlled to enterthe cross state by key bit 1. This causes the upper input of switch 122to be switched to the lower output of switch 122.

Receiver 106 also includes OCDMA decoder A 124 and OCDMA decoder B 126.OCDMA decoder A 124 outputs a positive signal (i.e., a large opticalpulse) if its input signal is an optical pulse encoded with code A, andoutputs a null signal (i.e., no large optical pulse) if its input signalis an optical pulse encoded with another code, such as code B.Similarly, OCDMA decoder B 126 outputs a positive signal if its inputsignal is an optical pulse encoded with code B, and outputs a nullsignal if its input signal is an optical pulse encoded with anothercode, such as code A. The outputs of OCDMA decoder A 124 and OCDMAdecoder B 126 feed into 2×1 coupler 128, which combines the outputs intoa single signal, and feeds the single signal to gate 130. Gate 130determines whether the signal corresponds to a data bit of 0 or 1.

In this example, switch 122 switches a code A signal, arriving at itsupper input, to its lower output and to OCDMA decoder B 126. OCDMAdecoder B 126 then outputs a null signal upon processing code A signal.Switch 122 further switches its lower input, which is empty, to itsupper output, which is connected to OCDMA decoder A 124. OCDMA decoder A124 outputs nothing because it receives no input signal. Thus, coupler128 couples nothing from OCDMA decoder A 124 with a null signal fromOCDMA decoder B 126, and outputs the null signal to gate 130. Gate 130maps a null signal to a data bit of 1 and a positive signal to a databit of 0. Because gate 130 receives a null signal in this example, itdetermines that the data bit is 1. In this way, system 100 encodes anoptical pulse with a code to represent a data bit, using a random key,such that only a receiver with the random key can determine the value ofthe data bit.

A drawback of system 100 is that a single encoded optical pulse from thelaser is limited to representing a single bit. It may be desirable for asingle encoded optical pulse to represent multiple bits, such as a dataword, in order to increase throughput.

SUMMARY

In accordance with the disclosed embodiments, there is provided a methodfor transmitting an encrypted data word of two or more bits, the methodcomprising the steps of: identifying a random key word comprising two ormore bits for encrypting the data word; identifying a set of uniqueorthogonal codes; selecting a code from the set of unique orthogonalcodes that corresponds to a result of an XOR operation between the twoor more bits of the data word and the random key word; encoding a signalwith the code; and transmitting the encoded signal as encrypted data.

In accordance with the disclosed embodiments, there is further provideda method for receiving an encrypted data word of two or more bits from atransmitter, the method comprising the steps of: identifying a set ofunique orthogonal codes; identifying a random key word comprising two ormore bits for decrypting a data word; receiving a signal encoded by afirst code of the set of unique orthogonal codes that corresponds to aresult of an XOR operation between the two or more bits of the data wordand the random key word; and selecting a second code, from the set ofunique orthogonal codes, on the basis of the two or more bits of therandom key word, the second code corresponding to the two or more bitsof the data word.

In accordance with the disclosed embodiments, there is further provideda system with a transmitter and a receiver for communicating anencrypted data word of two or more bits, wherein the transmitter isconfigured to: identify a random key word comprising two or more bitsfor encrypting the data word; identify a set of unique orthogonal codes;select a first code from the set of unique orthogonal codes thatcorresponds to a result of an XOR operation between the two or more bitsof the data word and the random key word; encode a signal with the firstcode; and transmit the encoded signal as encrypted data; and thereceiver is configured to: receive the signal encoded by the first code;and select a second code, from the set of unique orthogonal codes, onthe basis of the two or more bits of the random key word, the secondcode corresponding to the two or more bits of the data word.

It is important to understand that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only, and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments. In thedrawings:

FIG. 1 illustrates a prior-art system for encoding an optical pulse withone of two codes to encrypt a data bit.

FIGS. 2A and 2B illustrate properties of Hadamard codes, which may beused in the disclosed embodiments.

FIG. 3 illustrates a system for securely transmitting a data word as aHadamard code using electronic XOR-ing of the data word and random keyword to drive a fast dynamic encoder .

FIG. 4 illustrates a system for securely transmitting a data word as aHadamard code using a pair of dedicated dynamic encoders and the opticalXOR property of Hadamard codes.

FIG. 5 illustrates a system for securely transmitting a data word as aHadamard code using a pair of dedicated encoders, each based on acascade of static Hadamard encoders and fast optical switches.

FIG. 6 includes a system that allows an optical code to be selectedusing a pair of dedicated encoders, collectively including a pair ofstatic Hadamard encoders and fast optical switches.

FIG. 7 illustrates a system for transmitting multiple encrypted datawords on the same pulse using unique code sets.

DESCRIPTION OF THE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific techniques and embodiments are set forth, such asparticular sequences of steps, interfaces, and configurations, in orderto provide a thorough understanding of the techniques presented here.While the techniques and embodiments will primarily be described in thecontext of the accompanying drawings, those skilled in the art willfurther appreciate that the techniques and embodiments can also bepracticed in other electronic devices or systems.

Reference will now be made in detail to exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

As disclosed herein, the term “random” may refer to data that israndomly generated. For example, “random” may refer to a bit that has anequal likelihood of being a 1 or 0. As is known to one of ordinary skillin the art, in implementation, it may be difficult to generate data thatis truly random. For example, when a “random” bit is generated inpractice, it may be slightly more likely to be a 0 than a 1, or viceversa. This is due to practical constraints of random number generators.Accordingly, as disclosed herein, “random” may refer to data that ispseudo-randomly generated. Therefore, the term “random” may refer todata that is truly randomly generated and/or may refer to data thatpseudo-randomly generated.

FIGS. 2A and 2B illustrate properties of Hadamard codes, which may beused in the disclosed embodiments. As discussed, spectral phase encodinginvolves changing the relative phases of frequency lines in an opticalpulse in accordance with a set of codes. For the optical pulses producedby a phase-locked mode-locked laser (MLL), at the peak of the pulseintensity all of the frequency lines are in precise phase alignment(zero relative phase difference). The spectrum of the MLL may becomposed of N phase-locked continuous lines. The lines may be situatedon integer multiples of frequency spacing. This may give rise to a trainof optical pulses at a repetition rate corresponding to the frequencyspacing. The codes selectively determine, for the frequency lines,whether to change their phase, and if so, by how much. Some of the codeschange relative phases of the frequency lines and some codes may notchange any of the relative phases. Different codes can be used forspectral phase encoding, the different codes indicating differentfrequency lines to be phase shifted and by different amounts.

Hadamard codes are an example of these codes that are well known in theart. A set of Hadamard codes encompasses a closed orthogonal set. Aclosed orthogonal set is one such that a particular operation orcombination among any two members of the set yields a third member ofthe set, and not a member outside the set. For example, a combination ofany two Hadamard codes of a closed set results in a third Hadamard codethat is within the same set. Hadamard codes are also orthogonal, whichmeans that when a Hadamard code is combined with itself (or moregenerally, the complex conjugate of itself), it results in setting allof its relative phase shifts to 0 (effectively a unity operation). And,when a Hadamard code is combined with a different Hadamard code, therelative phase shifts are such that the optical intensity is zero at thetime at which the original optical pulse intensity would have been atits peak. In other words, different Hadamard codes do not interfere witheach other.

Although in a preferred embodiment a closed set of codes is used, it maybe possible to use a “unique” set of codes that has no repeating codesin the code set that may be either closed or non-closed.

A spectral phase encoder may encode an optical pulse with a firstHadamard code. The encoded optical pulse may then pass through adifferent spectral phase encoder and perform additional encoding with asecond Hadamard code from the same closed set. The resulting outputtedcoded optical pulse is encoded according to a combination of the firstHadamard code and the second Hadamard code, which is a third Hadamardcode in the same set. FIGS. 2A and 2B illustrate this property.

FIG. 2A illustrates a table 200 of a closed set of Hadamard codes oforder N=4. Table 200 would also apply to the first four Hadamard codesof a set of Hadamard codes of order higher than N=4. Column 202represents possible first Hadamard encoders used to encode an opticalpulse, ranging from H1-H4. Row 204 represents possible second Hadamardencoders used to further encode the optical pulse, also ranging fromH1-H4. Matrix 206 represents all combinations of an optical pulseencoded with both a first Hadamard code and a second Hadamard code.Matrix 206 ranges from H1-H4, the same range as both the first Hadamardcodes and the second Hadamard codes. Thus, the result of a combinationof any two Hadamard codes in the set H1-H4 stays in the set H1-H4.Another property is that a Hadamard code combined with itself yields thecode H1 which is the unity operator (introduces no relative phaseshifts). Moreover, if a first code combined with a second code yields athird code, then the third code can be combined with the second code toyield the first code. For example, from table 200, H2 combined with H3yields H4, and H4 combined with H3 yields H2 again. This property isuseful for encoding and decoding signals with Hadamard codes, as will bediscussed later.

In disclosed embodiments, column 202 is associated with the data word tobe sent and row 204 is associated with the random key used to encryptthe data word. Any of the four codes from matrix 206 that are ultimatelysent is equally likely to be associated with any of the four data words.Thus, if code H2 is sent, there is an equal likelihood of itcorresponding to H1, H2, H3 or H4 (depending on the random key word).

FIG. 2B illustrates table 208, which shows Hadamard codes H1-H4 writtenas their Walsh decompositions (i.e., products of the Walsh functions W1and W2), with H1=00, H2=01, H3=10, and H4=11 where the left digitindicates the presence or absence of W2 in the product and the rightdigit indicates the presence or absence of W1 in the product. Similartables can be constructed for higher-order Hadamard codes. Thus, H4 isformed of the product of both W1 and W2. Table 208 may be one way ofassociating each of Hadamard codes H1-H4 with a binary number to furtherillustrate properties of the Hadamard codes. The binary numbersassociated with Hadamard codes are not necessarily limited to theirWalsh decompositions. Indeed, in the disclosed embodiments, the binarynumbers associated with Hadamard codes may be arbitrary.

It can be seen from table 208 that combinations of Hadamard codes of aset can be viewed as a bit-by-bit XOR of their corresponding bits. Forexample, in FIG. 2A as discussed, encoding an optical pulse with H2 andH3 yields an optical pulse encoded with H4. Analogously, in FIG. 2B, avalue of 01 (mapping to H2) XORed with a value of 10 (mapping to H3)yields a value of 11 (mapping to H4). Moreover, by performing an XOR onthe resulting value 11 (H4) with 10 (H3) results in the original value01 (H2). Thus, encoding an optical signal with two Hadamard codes hasthe effect of performing a bit-by-bit XOR operation on data words andkey words that map to the Hadamard codes.

In this example, four possible Hadamard codes (i.e., H1-H4) map to twodata bits and two random key bits. More generally, in disclosedembodiments, the data word and the key word may each include log₂(N)number of bits, where N is the number of unique orthogonal codes, suchas Hadamard codes. For Hadamard codes, N is a power of 2.

In the Princeton University system illustrated in FIG. 1, a singleencoded optical pulse carried on an optical pulse represents only asingle bit of data. But it may be beneficial for a code to representmore than one bit. A “data word” is a grouping of bits of any number. Itmay be beneficial for a code to represent a data word of two or morebits to increase throughput, so that a code represents multiple bits.This may be implemented using a closed set of orthogonal codes, such asHadamard codes.

FIG. 3 illustrates a system 300 for securely transmitting a data word asa Hadamard code using an electronic XOR followed by a fast dynamicencoder (one capable of changing its encoding state on the time scale ofa single optical pulse). For the purposes of explanation, FIG. 3 (aswell as the subsequent Figures) uses a closed Hadamard set of order 4(Le., H1-H4). However, any set of codes that are, for example, uniquefrom each other and orthogonal to each other may be used.

FIG. 3 includes a transmitter 302 for transmitting an encrypted dataword of two or more bits and a receiver 304 for receiving the encrypteddata word of two or more bits from the transmitter 302. Transmitter 302and/or receiver 304 may include fast dynamic spectral phase encoder(s)that can rapidly encode an optical pulse with any number of codes, suchas Hadamard codes.

For the purposes of example, it is assumed that a data word fortransmission and a random key word for encryption each have a length oftwo bits. A set of Hadamard codes of order 4 (e.g., H1-H4) may be usedto represent any of the four combinations of the two bits. Moreover,H1-H4 may be mapped to bit combinations according to their Walshdecompositions, as illustrated in FIGS. 2A and 2B. It is further assumedin this example that the data word=01 and the key word=10. The number ofbits and values are exemplary only.

Transmitter 302 may include component 306, which may be a computingunit, such as a processor or dedicated semiconductor component, thatelectronically calculates an XOR between data word 01 and random keyword 10. The result of the XOR is 11. The value 11 may map to Hadamardcode H4, according to the mapping in FIGS. 2A and 2B.

Transmitter 302 further includes optical pulses 308 from a mode-lockedlaser (MLL) and encoder 310. Encoder 310 receives optical pulses 308 andencodes one with code H4 on the basis of the XOR operation at component306. Transmitter 302 sends the optical pulse encoded by H4 to receiver304.

Receiver 304 includes encoder 312, which may receive the optical pulseencoded by H4. Encoder 312 further encodes the received optical pulse308 with a Hadamard code selected according to the shared secret keyword 10. Using the mapping from FIGS. 2A and 2B, bits 10 correspond toH3. Thus, optical pulse 308 encoded by H4 is further encoded by H3.Using the mapping from FIG. 2A, an optical pulse encoded with both H4and H3 yields an optical pulse encoded by H2. H2 corresponds to bits 01,which is the originally encoded data bit.

In order to determine that the output of encoder 312 is H2, receiver 304includes a bank of decoders 314. Decoders 314 may be spectral phasedecoders, such as Hadamard decoders. Decoders 314 may include separatedecoders for H1-H4. The decoder for H3 outputs a positive signal if itsinput is also H3, and outputs a null for all other inputs. The decodersfor H1, H2, and H4 operate similarly. Thus, encoder 312 outputs anoptical pulse encoded with H3. This optical pulse goes through decodersH1, H2, and H4, which yield a null output, and decoder H3, which yieldsa positive output. Because only decoder H3 yields a positive result, theoptical pulse is deemed to be encoded with H3. H3 can then be mapped todata word 01 using a known mapping. In this way, system 300 encryptsdata bits 01 with key word 10 at transmitter 302 and decrypts data bits01 with key word 10 at receiver 304.

If the bank of decoders are Hadamard decoders, N decoders will berequired (where N is the number of possible different codes sent by thetransmitter). Alternatively, the decoders may be Composite Phase Masks(CPMs) instead of Hadamard decoders 314 and only log₂(N) of such CPMdecoders will suffice. CPMs are disclosed in the paper “Direct OpticalProcessing of M-ary Code-Shift Keyed OCDMA using Integrated PassiveOptical Phase Decoders,” by Menendez et al., the contents of which arehereby incorporated by reference. Composite phase masks directly outputthe actual bit from the encoded optical pulse, and may include onecomposite phase mask per bit. Thus, while using decoders 314 may requirefour separate Hadamard decoders, using composite phase masks may onlyrequire two separate composite phase masks.

In FIG. 3, the bit-by-bit XOR operation between data word 01 and keyword 10 is performed electronically. In some disclosed embodiments, thisoperation may be performed optically.

FIG. 4 illustrates a system 400 for securely transmitting a data word asa Hadamard code using an optical XOR. FIG. 4 includes a transmitter 402for transmitting an encrypted data word of two or more bits and areceiver 404 for receiving the encrypted data word of two or more bitsfrom transmitter 402. Transmitter 402 and/or receiver 404 may includefast dynamic spectral phase encoder(s) that can rapidly encode anoptical pulse with any number of codes, such as Hadamard codes.

Transmitter 402 receives MLL optical pulses 406 at encoder 408. Encoder408 selects code H2 based on data word 01 and spectrally phase encodesthe optical pulse with H2. Encoder 410 receives the optical pulseencoded with H2. Encoder 410 selects code H3 based on key word 10 andfurther encodes the optical pulse encoded by H2 with H3. Encoder 410outputs an optical pulse encoded by H2 and H3, in other words, anoptical pulse encoded by H4 (which is the combination of H2 and H3, asshown in FIG. 2A). This encoding with H2 (corresponding to 01) and H3(corresponding to 10) to yield H4 (corresponding to 11) is an opticalXOR that replaces the electronic XOR performed by component 306 in FIG.3. Indeed, both transmitter 302 from FIG. 3 and transmitter 402 fromFIG. 4 output an optical pulse encoded with H4, using data word 01 andkey word 10.

Transmitter 402 sends the optical pulse encoded with H4 to receiver 404.Receiver 404 includes encoder 412 and decoders 414, which are similar toencoder 312 and decoders 314 from FIG. 3. Moreover, receiver 404 in FIG.4 may perform the same function as receiver 304 in FIG. 3, and will notbe discussed further.

In some cases, fast dynamic spectral phase encoders may not beavailable. Accordingly, the disclosed embodiments may be implementedusing a cascade of static spectral phase encoders with fast opticalswitches. The static spectral phase encoders may only be able to phaseencode with a single code, for example, a single Hadamard code.

FIG. 5 illustrates a system 500 for securely transmitting a data word asa Hadamard code using dedicated Hadamard coders. Each dedicated Hadamardcoder may be configured to encode a static Hadamard code. The componentsfrom system 500 map to the components from system 400, which are alsoshown in FIG. 5.

In particular, optical encoding device 502 performs the function ofencoder 408 from system 400, and optical encoding device 504 performsthe function of encoders 410 and 412 from system 400.

Optical encoding device 502 includes a 2x2 switch 506 that receives MLLpulses on its upper input. In this example, the lower input of switch506 may have no signal and is ignored. Switch 506 is controlled by firstdata bit 0 of data word 01 to enter into a bar state. Therefore, switch506 switches its upper input to its upper output, bypassing encoder 508.Encoder 508 may be a dedicated Hadamard encoder for code H4. But theoutput of switch 506 reaches 2×2 switch 510 without passing throughencoder 508. Switch 510 is controlled by a second data bit 1 of dataword 01 to enter into a cross state. Therefore, switch 510 switches itsupper input to its lower output towards encoder 512. Encoder 512 may bea dedicated Hadamard encoder for code H2. Encoder 512 encodes the loweroutput of switch 510 with H2 and outputs the encoded pulse to 2x2coupler 514. Coupler 514 combines its lower input with the encodedoptical pulse with its upper input of no signal, and outputs thecombination on both outputs. The lower output of coupler 514 is ignored,and the upper output is transmitted to optical encoding device 504.

Encoder 504 includes 2×2 switch 516, which receives the optical pulseencoded with H2 on its upper input. For this example, the lower input isignored. Switch 516 is controlled by first key bit 1 of key word 10 toenter into a cross state. Therefore, switch 516 switches its upper inputto its lower output toward encoder 518. Encoder 518, similar to encoder508, may be a dedicated Hadamard encoder for code H4. Encoder 518encodes the optical pulse encoded with H2 further with H4, and outputsan optical pulse encoded by the product H2 and H4. In other words,encoder 518 outputs an optical pulse encoded with H3 (a combination ofH2 and H4). Encoder 518 outputs this optical pulse to switch 520. Switch520 is controlled by a second key bit 0 of key word 10 to enter into abar state. Therefore, switch 520 switches its lower input to its loweroutput towards encoder 522. Encoder 522, similar to encoder 512, may bea dedicated Hadamard encoder for code H2. Encoder 522 encodes theoptical pulse encoded with H3 further with H2, and outputs an opticalpulse encoded by the product of H3 and H2. In other words, encoder 522outputs an optical pulse encoded with H4 (a combination of H3 and H2).This optical pulse is sent to a receiver by coupler 524, which mayoperate similar to coupler 514. The receiver may include opticalencoding device 504 to undo the effect of key word 10. The receiver maythen determine the data word in a similar manner, as illustrated inFIGS. 3 and 4.

A drawback to the system in FIG. 5 is that the transmitter may encode anoptical pulse with the same code in several different ways. For example,in FIG. 5, the transmitter could output an optical pulse encoded with H4by encoding the pulse in the following ways: through only encoder 508;through only encoder 518; through only encoders 508, 512, and 522, etc.Moreover, different combinations of encoders leave different signatureson an encoded optical pulse. In this example, even though thetransmitter outputs H4, there are different ways of obtaining H4, andeach way may leave a different signature in the encoded pulse thatcarries H4. An intercepting user may be able to analyze these signaturesto determine the makeup of optical encoding devices 502 and 504. Thiswould compromise the security of system 500.

FIG. 6 includes a system 600 that allows an optical code to be selectedusing only a single combination of encoders. This may reduce any abilityfor an intercepting party to analyze signatures in a transmitter output.The components from system 600 map to the components from system 400,which are also shown in FIG. 6.

In particular, optical encoding device 602 performs the functions ofencoders 408 and 410 from system 400. Optical encoding device 602 mayinclude 2x2 switches that are controlled by two input bits. The switchesmay enter into a bar state if the XOR between the bits results in a 0(e.g., 0 XOR 0, 1 XOR 1), and may enter into a cross state if the XORbetween the bits results in a 1 (e.g., 1 XOR 0, 0 XOR 1).

In particular, optical encoding device 602 includes a 2×2 switch 604that receives MLL pulses on its upper input. In this example, the lowerinput of switch 604 may have no signal and is ignored. Switch 604 iscontrolled by a first data bit 0 (of data word 01) and a first key bit 1(of key word 10), the XOR of which results in a 1. Thus, switch 604enters into a cross state. Switch 604 switches its upper input to itslower output towards encoder 606. Encoder 606 may be a dedicatedHadamard encoder for code H4. Encoder 606 may receive the lower outputfrom switch 604 as an optical pulse and encode the optical pulse withcode H4. Encoder 606 may output the encoded optical pulse to switch 608.

Switch 608 is controlled by a second data bit 1 (of data word 01) and asecond data bit 0 (of key word 10), the XOR of which results in a 1.Thus, switch 608 enters into a cross state. Switch 608 switches itslower input to its upper output, in this case, causing the opticalsignal to bypass encoder 610. Encoder 610 may be a dedicated Hadamardencoder for code H2. Coupler 612 receives the optical pulse encoded withH4 on its upper input and combines it with its lower input of no signal,and outputs the combination on both outputs to a receiver. The receivermay include an encoder, similar to optical encoding device 504 from FIG.5, and may decode the optical pulse encoded with H4 in a similar manneras in FIGS. 3-5. Note that the encoder 412 is driven only by the sharedkey word.

In a disclosed embodiment, the systems of FIGS. 5 and 6 may be builtwith 2×1 switches instead of 2×2 switches. This can be implemented usinga 2×1 coupler before the second cascaded switch. For example, in FIG. 6,switches 604 and 608 may be replaced by 2×1 optical switches, and a 2×1coupler may be inserted before switch 608 to ensure that it receivesonly a single input. However, 2×1 switches may be lossier than 2×2switches.

It may be possible to send two data words on the same pulse. Forexample, two different sets of orthogonal codes may be identified. Thecodes within each of the sets may be unique from all other codes in theset, and from all codes in the other set. The sets may be closed sets.Different data words may be encoded with the different data sets andsent on the same optical pulse.

As discussed, the examples shown in FIGS. 5 and 6 use the closed set ofHadamard codes H1-H4. But disclosed embodiments are not limited toclosed sets, and can also include open sets, so long as a transmittedcode is equally likely to correspond any data word without knowledge ofa random key word used for encryption.

One way of implementing a non-closed set is to modify the encoders inFIGS. 5 and 6 by adding a third static spectral phase encoder. The thirdcode implemented by the third static spectral phase decoder is chosenfrom codes not already produced by the unmodified encoder.

For example, the open set of Hadamard codes H5-H8 may be used in FIGS. 5and 6, by adding an H5 spectral phase encoder. The codes H5-H8 are anopen set (i.e., non-closed) because a combination of two codes within aset can yield a code outside the set (e.g., H5*H6 yields H2). Code H5 isnot implemented by any of the static spectral phase encoders in FIGS. 5and 6. In encoder 502, the H5 encoder may be placed on the upper inputof switch 506 in FIG. 5 or on the upper output of coupler 514. Or the H5encoder may be placed on the upper input of switch 510 and spectralphase encoder 508 is changed to a H4*H5 encoder (i.e., H8).Alternatively, the H5 encoder may be placed on the upper input ofcoupler 514 and spectral phase encoder 512 is changed to a H2*H5 encoder(i.e., H6). Similar modification may be made to encoder 504 in FIG. 5and encoder 602 from FIG. 6.

The use of H5-H8 as an open set is exemplary only. Other open(non-closed) sets can be used, such as {H1, H3, H5, H7} and {H2, H4, H6,H8}. Moreover, there are many other schemes of using non-closed setsthat may be used in disclosed embodiments, and the examples above arefor illustrative purposes only. There can also be more than two groupsof sets. For example, the closed set Hadamard 16 can be divided into 2groups of 8 codes or 4 groups of 4 codes.

FIG. 7 illustrates a system 700 for simultaneously transmitting multipleencrypted data words on the same pulse using unique disjoint code sets.FIG. 7 includes transmitters 702 and 704. Transmitter 702 selects a codefrom code set A, according to a first data word dl and a first key wordr1, and may code a first signal. Transmitter 704 selects a code fromcode set B, according to a second data word d2 and a second key word r2,and codes a second signal. The first and second signal may be opticalsignals and may the same or different. In some embodiments, the firstand second signal may be copies of each other. First and second keywords r1 and r2 may be independent. Moreover, code sets A and B may beunique and different from each other. In other words, code sets A and Bdo not share any codes. Moreover in a preferred embodiment, all of thecodes in code sets A and B are orthogonal with each other.

Transmitters 702 and 704 each output a differently coded optical pulseand may send them to a 2x1 coupler 706 to bit-synchronously combinethem. Coupler 706 may send the combined pulses over a single opticalfiber. The separate codes may obscure each other and make it moredifficult for an intercepting party to determine the codes being sent.This may add another layer of security.

The optical pulse over the single fiber may be split at the receivingend by a 1×2 splitter 708. One of the signals may be sent to receiver710 and the other to receiver 712. Receivers 710 and 712 may receive theidentical or substantially similar combined signal. Receivers 710 and712 may decode their respective signals. In particular, receiver 710 mayseek to recover first data word d1 by selecting a code from code set Aaccording to first key word r1. Even though receiver 710 receives anoptical pulse encoded with two codes, one from code set A and one fromcode set B, receiver 710 is still able to recover the code from code setA intended for it. This is because the codes from code set B do notinterfere with the codes from code set A, due to their orthogonality.

Similarly, receiver 712 may seek to recover second data word d2 byselecting a code from code set B according to second key word r2. Eventhough receiver 712 receives an optical pulse encoded with two codes,one from code set A and one from code set B, receiver 712 is still ableto recover the code from code set B intended for it. This is because thecodes from code set A do not interfere with the codes from code set B,due to their orthogonality. Receivers 710 and 712 may use banks ofeither decoders or composite phase masks to determine their respectivedata word.

The foregoing description has been presented for purposes ofillustration. It is not exhaustive and does not limit the invention tothe precise forms or embodiments disclosed. Modifications andadaptations of the invention can be made from consideration of thespecification and practice of the disclosed embodiments of theinvention. For example, one or more steps of methods described above maybe performed in a different order or concurrently and still achievedesirable results.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope of theinvention being indicated by the following claims.

1. A method for transmitting an encrypted data word of two or more bits,the method comprising the steps of: identifying a random key wordcomprising two or more bits for encrypting the data word; identifying aset of unique orthogonal codes; selecting a code from the set of uniqueorthogonal codes that corresponds to a result of an exclusive-or (XOR)operation between the two or more bits of the data word and the randomkey word; encoding a signal with the code; and transmitting the encodedsignal as encrypted data.
 2. The method of claim 1, wherein the set ofunique orthogonal codes comprises a number N of Hadamard codes.
 3. Themethod of claim 2, wherein the bits of the data word and the bits of therandom key word each include log₂(N) number of bits.
 4. The method ofclaim 1, wherein: the signal is an optical signal; and the step ofencoding the signal further comprises the step of encoding the opticalsignal with the code at an optical encoding device, the optical encodingdevice including at least one spectral phase encoder.
 5. The method ofclaim 4, wherein the step of encoding the signal further comprises thestep of: changing, with the at least one spectral phase encoder, phasesof the frequency lines of the optical signal in accordance with thecode.
 6. The method of claim 4, wherein the optical encoding deviceincludes at least one optical switch, and wherein the step of encodingthe signal further comprises the steps of: receiving, at the at leastone optical switch, the optical signal; receiving, at the at least oneoptical switch, at least one of the bits of the data word or one of thebits of the random key word to control switching of the optical signal;and switching the optical signal to the spectral phase encoder to encodethe optical signal with the code.
 7. The method of claim 1, wherein thestep of selecting a code further comprises the step of: opticallyperforming the XOR operation to select the code with at least twooptical switches and at least two spectral phase encoders that eachencode a different static Hadamard code.
 8. The method of claim 1,wherein the data word is a first data word, the random key word is afirst random key word, the set of unique orthogonal codes is a first setof unique orthogonal codes, the signal is a first signal, and the codeis a first code, the method further comprising the step of: selecting asecond code from a second set of unique orthogonal codes, unique fromthe first set of unique orthogonal codes, the second code correspondingto a result of an exclusive-or (XOR) operation between bits of a seconddata word and bits of a second random key word; and wherein the step ofencoding a signal further comprises the step of encoding the firstsignal with the first code and a second signal with the second code, fora combined first signal and second signal to carry both the first dataword and the second data word, and to obscure the first code and thesecond code.
 9. The method of claim 1, wherein the set of uniqueorthogonal codes is a closed set.
 10. A method for receiving anencrypted data word of two or more bits from a transmitter, the methodcomprising the steps of: identifying a set of unique orthogonal codes;identifying a random key word comprising two or more bits for decryptinga data word; receiving a signal encoded by a first code of the set ofunique orthogonal codes that corresponds to a result of an exclusive-or(XOR) operation between the two or more bits of the data word and therandom key word; and selecting a second code, from the set of uniqueorthogonal codes, on the basis of the two or more bits of the random keyword, the second code corresponding to the two or more bits of the dataword.
 11. The method of claim 10, wherein the set of unique orthogonalcodes comprises a number N of Hadamard codes.
 12. The method of claim11, wherein the bits of the data word and the bits of the random keyword each include log₂(N) number of bits.
 13. The method of claim 10,wherein the signal is an optical signal, further comprising the step of:decrypting the signal encoded by the first code into a signal encoded bythe second code at an optical encoding device, the optical encodingdevice including at least one spectral phase encoder.
 14. The method ofclaim 13, wherein the step of decrypting the signal further comprises:receiving, at the at least one spectral phase encoder, the opticalsignal encoded by the first code; changing phases of the frequency linesof the optical signal encoded by the first code, in accordance with acode of the at least one spectral phase encoder; and outputting, at theat least one spectral phase encoder, the optical signal encoded by thesecond code.
 15. The method of claim 13, wherein the optical encodingdevice includes at least one optical switch, and wherein the step ofdecrypting the signal further comprises the steps of: receiving, at theat least one optical switch, the optical signal encoded by the firstcode and at least one bit of the random key word to control switching ofthe optical switch; and switching the optical signal encoded by thefirst code with the at least one optical switch to the spectral phaseencoder.
 16. The method of claim 10, further comprising the step of:receiving the signal encoded by the second code at composite phasemasks, each of the composite phase masks associated with a bit of thedata word; and outputting, at the composite phase masks, the bits of thedata word.
 17. The method of claim 10, further comprises the steps of:receiving the signal encoded by the second code at Hadamard decoders,each of the Hadamard decoders associated with a different code from theclosed set of predefined orthogonal codes; receiving a first output fromone of the Hadamard decoders and a second output from all remainingHadamard decoders; determining that the signal is encoded by the secondcode by identifying the Hadamard decoder with the first output; anddetermining the data word by mapping the second code to the data word.18. The method of claim 10, wherein the set of unique orthogonal codesis a closed set.
 19. A system with a transmitter and a receiver forcommunicating an encrypted data word of two or more bits, wherein: thetransmitter is configured to: identify a random key word comprising twoor more bits for encrypting the data word; identify a set of uniqueorthogonal codes; select a first code from the set of unique orthogonalcodes that corresponds to a result of an exclusive-or (XOR) operationbetween the two or more bits of the data word and the random key word;encode a signal with the first code; and transmit the encoded signal asencrypted data; and the receiver is configured to: receive the signalencoded by the first code; and select a second code, from the set ofunique orthogonal codes, on the basis of the two or more bits of therandom key word, the second code corresponding to the two or more bitsof the data word.
 20. The system of claim 19, wherein the set of uniqueorthogonal codes comprises a number N of Hadamard codes.
 21. The systemof claim 20, wherein the bits of the data word and the bits of therandom key word each include log₂(N) number of bits.
 22. The system ofclaim 19, wherein the transmitter comprises an optical encoding device,with at least one spectral phase encoder, configured to: receive thesignal as an optical signal; and encode the optical signal with thefirst code.
 23. The system of claim 22, wherein the spectral phaseencoder is configured to: change phases of the frequency lines of theoptical signal in accordance with to the first code.
 24. The system ofclaim 22, the optical encoding device further including at least oneoptical switch, the optical switch configured to: receive the opticalsignal; receive at least one of the bits of the data word or one of thebits of the random key word to control switching of the optical signal;and switch the optical signal to the spectral phase encoder to encodethe optical signal with the first code.
 25. The system of claim 19,wherein the transmitter comprises at least two optical switches and atleast two spectral phase encoders collectively configured to: opticallyperform the XOR operation to select the first code, wherein the spectralphase encoders each encode a different static Hadamard code.
 26. Thesystem of claim 19, wherein the receiver comprises an optical encodingdevice, including at least one spectral phase encoder, configured to:receive the signal as an optical signal; and decrypt the optical signalencoded by the first code into a signal encoded by the second code. 27.The system of claim 26, wherein the spectral phase encoder is configuredto: receive the optical signal encoded by the first code; change phasesof the frequency lines of the optical signal encoded by the first code,according to a code of the spectral phase encoder; and output theoptical signal encoded by the second code.
 28. The system of claim 26,the optical encoding device further comprising at least one opticalswitch, the optical switch configured to: receive the optical signalencoded by the first code and at least one bit from the random key wordto control switching of the optical switch; and switch the opticalsignal encoded by the first code to the spectral phase encoder.
 29. Thesystem of claim 19, wherein the receiver comprises composite phase maskseach associated with a bit of the data word, the composite phase masksconfigured to: receive the signal encoded by the second code; andoutputting the bits of the data word.
 30. The system of claim 19,wherein the receiver comprises Hadamard decoders configured to: receivethe signal encoded by the second code; and output a first output fromone of the Hadamard decoders and a second output from all remainingHadamard decoders, wherein the receiver is further configured todetermine the second code by identifying the Hadamard decoder with thefirst output, and determine the data word by mapping the second code tothe data word.
 31. The system of claim 19, wherein the data word is afirst data word, the random key word is a first random key word, the setof unique orthogonal codes is a first set of unique orthogonal codes,the signal is a first signal, and the code is a first code, wherein thetransmitter is further configured to: select a second code from a secondset of unique orthogonal codes, unique from the first closed set ofpredefined orthogonal codes, the second code corresponding to a resultof an exclusive-or (XOR) operation between bits of a second data wordand bits of a second random key word; and encode the first signal withthe first code and the second signal with the second code, for acombined first signal and second signal to carry both the first dataword and the second data word.
 32. The system of claim 19, wherein theset of unique orthogonal codes is a closed set.